Yay !
Indeed the key is always to look for your own answer "Why", because at the end of the day, mechanics really only tell you the "what".
Let me dip back on some real life knowledge for another reason why a deck is so much more pricey than a commlink.
It is to do with what is called 'transmission security'. Obviously for a device to participate on the wireless matrix, it must transmit. But it just isn't as simple as "hey, lets broadcast on this frequency."
Zillions of devices all have to use the matrix at once, and they do so through various techniques, such as time division multiplexing, and frequency separation and/or sharing, and all sorts of goodies.
What that means is, at any given moment, a commlink could be transmitting on any one of a whole range of frequencies. And the pattern that it uses can be unique and specific, and if you don't know that pattern, you cant even begin to listen or communicate with the device, as you don't know where to send your transmission. That's transmission security, and as a decker, you want to be able to overcome that.
So a deck has to be able to interact simultaneously with loads of frequencies all at once, cast a big net as it were, in order to catch your commlink.
Which means that a deck has to have the transmission part of many commlinks. It is a parallel commlink, and thus right off the bat, expensive as several commlinks because it is several commlinks.
Do you need to know this? Not at all. The rules say that a commlink is a couple of grand and a deck is a few hundred grand. But sometimes you want to know, even if you don't need to. So you think about the why, and come up with your own reason. Maybe based on real world stuff, maybe based on logic and extrapolation, or maybe pseudoscientific gobbledygook or the phrase "Its magic."
The important thing is though - don't be asking "why is this the rule", because it just is. Instead ask "What fits the rule".
I wonder if anyone will understand what I mean here? Oh well.